Methods And Systems For Bandwidths Doubling In An Ethernet Passive Optical Network

ABSTRACT

Systems and methods for bandwidth doubling in an Ethernet passive optical network (EPON) enable an optical line terminal (OLT) to transmit downlink to at least one double rate optical network unit (ONU). The double rate transmission is preferably facilitated by use of single rate devices (OLT and ONU) functionally connected to provide the double rate capability. The methods include packet-by-packet multiplexing, bit-by-bit line code interleaving, doubling an inter-packet gap (IPG) length, defining windows of transmission for different transmission rates, using the 8B/10B code, removing the 8B/10B code from just the downlink transmission and symbol-by-symbol multiplexing is downlink transmissions from the double rate OLT.

FIELD OF THE INVENTION

The present invention relates generally to passive optical networks(PON), and more particularly to methods and system for increasingdownlink transmission rates in Ethernet passive optical networks (EPON).

BACKGROUND OF THE INVENTION

Ethernet passive optical networks are known, as described for example inUS Patent Application No. 20020196801 to Haran et al. In general, asshown in FIG. 1, an EPON 100 comprises an optical link terminal (OLT)102 connected through a splitter 104 to a plurality (in this case 3) ofoptical network units (ONUs) 106, 108 and 110. The OLT transmitsdownlink transmissions to each ONU. Each ONU transmits uplinktransmissions to the OLT. In a “unified” EPON, all devices transmit atthe same rate, for example currently at 1 Gigabit per second (1 G).

The new IEEE 802.3 EPON standard specifies a 1 G access network sharedbetween multiple users. The current definition of the uplink/downlinkchannels is symmetric. Looking forward, the existing bandwidth (BW)might not be sufficient when customer BW requirements will increase, forexample when Internet protocol (IP) high-definition television (HDTV)services become popular.

A time-dependent channel is a communication channel in which the dataarrival time plays as important a role as the data content itself.Examples for such channels are:

-   -   Control messages with embedded timestamps, as in IEEE802.3 EPON    -   A channel with encryption based on packet transmission time, as        commonly implemented in EPON solutions

Doubling of a downlink channel (path) BW can be performed simply by timedivision multiplexing (TDM), as well known in the art. In this case, twoindependent channels are used, placing on the transmission lineinformation from one channel followed by information from the otherchannel, based on a selected scheme. The problem with this type ofsolution is that the line code appears as two normal rate (1 G) links,not one double rate (2 G) link.

It would therefore be advantageous to have mechanisms required forhigher rate downlink transmission support in EPON. In particular, itwould be advantageous to have a 2 G downlink BW in EPON. It would befurther advantageous that these mechanisms retain maximal similarity tothe existing 1 G solution, thereby allowing fast time-to-market.

SUMMARY OF THE INVENTION

The present invention discloses methods and systems for doubling an EPONdownlink transmission to rates up to 2 G from the existing 1 G(“dual-rate operation” of 1 G and 2 G). The speed-up is achieved invarious ways, e.g. by providing a line coding in which symbols are runtwice faster than in existing coding, or by bit interleaving with a bitalignment to 20 bits. In addition, the present invention provides anability to add delay information to a packet preamble, thereby achievinga solution that can use two existing 1 G components for creating the 2 Glink.

According to the present invention there is provided a method forbandwidth doubling in an EPON comprising the steps of providing a mixedsystem in which a logical OLT is optically coupled to at least onesingle rate ONU and to at least one double rate ONU, wherein the OLT isoperative to transmit downstream packets through a downlink to each ONU,and wherein each ONU is operative to transmit upstream packets throughan uplink to the OLT; and enabling the OLT to transmit to each ONU at adownstream rate that matches the respective ONU rate.

According to one feature of the method of the present invention, thestep of providing a mixed system in which a logical OLT is opticallycoupled to at least one single rate ONU and to at least one double rateONU includes providing a mixed system in which each single rate ONU is a1 G ONU and each double rate ONU is a 2 G ONU operative to performcombined 1 G and 2 G operations.

According to another feature of the method of the present invention, thestep of providing a mixed system in which a logical OLT is opticallycoupled to at least one single rate ONU and to at least one double rateONU includes providing a mixed system in which the logical OLT is a 2 GOLT that comprises a functional combination of two 1 G OLT devices andis operative to transmit to each 2 G ONU at a 2 G rate.

According to the present invention, in a first embodiment of the method,the step of enabling the 2 G OLT to transmit to each 2 G ONU at a 2 Grate includes packet-by-packet multiplexing of packets from the two 1 GOLT devices, and outputting a 2 G downstream traffic output in a 2 Gchannel.

According to the present invention, in a second embodiment of themethod, the step of enabling the 2 G OLT to transmit to each 2 G ONU ata 2 G rate includes line code interleaving two different 1 G links intoa single 2 G downlink.

According to the present invention, a particular feature of the linecode interleaving includes bit-by-bit interleaving of two differentbytes starting with the least significant bit (LSB) of each byte; andspacing the two different bytes with a separation of a byte therebetweenin a transmission stream.

According to the present invention, in a third embodiment of the method,the step of enabling the 2 G OLT to transmit to each 2 G ONU at a 2 Grate includes doubling an inter-packet gap (IPG) length in the downlinktransmission of packets to each double rate ONU, whereby the doubling ofthe IPG guarantees comma synchronization.

According to the present invention, in a fourth embodiment of themethod, the step of enabling the 2 G OLT to transmit to each 2 G ONU ata 2 G rate includes defining transmission windows, each windowtransmission occurring at a defined rate.

According to the present invention, a particular feature of the definingof transmission windows that transmit at a defined rate includesperforming reordering of the downlink transmission by placing eachpacket in a respective queue according to a required transmission ratein an ingress process, selecting a group of queues for transmission inan egress process, and transmitting the packets in each queue accordingto results of the selection.

According to the present invention, in a fifth embodiment of the method,the step of enabling the 2 G OLT to transmit to each 2 G ONU at a 2 Grate includes configuring the OLT to transmit to each 2 G ONU at a 2 Grate using a 8 B/10 B line code.

According to the present invention, in a sixth embodiment of the method,the step of enabling the 2 G OLT to transmit to each 2 G ONU at a 2 Grate includes removing a 8 B/10 B code from the downstream transmission.

According to the present invention, in a seventh embodiment of themethod, the step of enabling the 2 G OLT to transmit to each 2 G ONU ata 2 G rate includes symbol-by-symbol multiplexing.

According to the present invention there is provided a method forbandwidth doubling in an EPON comprising the steps of providing a mixedsystem in which an OLT is optically coupled to at least one 1 G ONU andto at least one 2 G ONU, wherein the OLT is operative to transmitdownstream packets through a downlink to each ONU, and wherein each ONUis operative to transmit upstream packets through an uplink to the OLTand configuring the OLT to transmit to each 2 G ONU at a 2 G rate.

According to the present invention there is provided a method forbandwidth doubling in an EPON comprising the steps of providing a mixedsystem in which an OLT is optically coupled to at least one 1 G ONU andto at least one 2 G ONU, wherein the OLT is operative to transmitdownstream packets through a downlink to each ONU, and wherein each ONUis operative to transmit upstream packets through an uplink to arespective OLT; and by the OLT, performing packet-by-packet multiplexingof packets and outputting a 2 G downstream traffic output in a 2 Gchannel.

According to the present invention there is provided a method forbandwidth doubling in an EPON comprising the steps of providing a mixedsystem in which an OLT is optically coupled to at least one 1 G ONU andto at least one 2 G ONU, wherein the OLT is operative to transmitdownstream packets through a downlink to each ONU, and wherein each ONUis operative to transmit upstream packets through an uplink to the OLT,and, by the OLT, line code interleaving two different 1 G links into asingle 2 G downlink.

According to the present invention there is provided a method forbandwidth doubling in an EPON comprising the steps of providing a mixedsystem in which an OLT is optically coupled to at least one 1 G ONU andto at least one 2 G ONU, wherein the OLT is operative to transmitdownstream packets through a downlink to each ONU, and wherein each ONUis operative to transmit upstream packets through an uplink to the OLT,and, by the OLT, doubling an inter-packet gap (IPG) length in thedownlink transmission of packets to each double rate ONU, whereby thedoubling of the IPG guarantees comma synchronization at the 1 G ratebefore the downlink transmission to each 1 G ONU.

According to the present invention there is provided a method forbandwidth doubling in an EPON comprising the steps of providing a mixedsystem in which an OLT is optically coupled to at least one 1 G ONU andto at least one 2 G ONU, wherein the OLT is operative to transmitdownstream packets through a downlink to each ONU, and wherein each ONUis operative to transmit upstream packets through an uplink to the OLT,and, by the OLT, defining transmission windows, wherein each window'stransmission occurs at a defined rate selected from the group consistingof a 1 G rate and a 2 G rate.

According to the present invention there is provided a system foreffecting bandwidth doubling in an EPON that comprises a logical OLToptically coupled to a plurality of ONUs, the system comprising amechanism for doubling the downlink transmission rate from each OLT toat least one ONU configured to receive double rate transmissions.

According to one feature in the system for effecting bandwidth doublingin an EPON of the present invention, the mechanism for doubling thedownlink transmission rate from each OLT to at least one ONU comprises asubsystem comprising two single rate OLT devices functionally connectedthrough a complex programmable logic device to provide a double ratetransmission functionality to the logical OLT.

According to another feature in the system for effecting bandwidthdoubling in an EPON of the present invention, the single rate is a 1 Grate and the double rate is a 2 G rate.

According to yet another feature in the system for effecting bandwidthdoubling in an EPON of the present invention, the at least one ONUconfigured to receive double rate transmissions is a 2 G ONU thatcomprises two 1 G ONU devices functionally connected to receive 2 Gdownlink transmissions.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention and to show moreclearly how it could be applied, reference will now be made, by way ofexample only, to the accompanying drawings in which:

FIG. 1 shows schematically an EPON;

FIG. 2 shows a subsystem of two OLT 1 G components, connected togetherto achieve a 2 G link in an EPON of the present invention;

FIG. 3 shows a subsystem of two ONU 1 G components, connected togetherto achieve a 2 G link in an EPON of the present invention;

FIG. 4 shows an embodiment of packets spaced by double the inter-packetgap (IPG) in order to allow simultaneous comma synchronization in tworates;

FIG. 5 a shows a flow chart of a scheme to reorder packet transmissionbased on their transmission rate;

FIG. 5 b shows schematically a subsystem in which the reordering processof FIG. 5 a is carried out;

FIG. 6 shows schematically a basic packet-by-packet (PBP) multiplexingsystem;

FIGS. 7 a, b illustrate the process of PBP multiplexing: a) without timedelay; b) with time delay

FIG. 8 shows an exemplary summary of two bytes placed in a packetpreamble for error correction.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention discloses a “mixed” EPON system that comprisesmixed end devices, some supporting a basic operating rate and somesupporting a higher operating rate in the downlink between an OLT andeach ONU. A mixed EPON requires the higher (e.g. double) rate to be amultiplicity of the basic clock rate. Such a multiplicity is alsorequired in the uplink rate, which is derived from the downlink rate. Ifthe higher rate is not a multiplicity of the basic clock rate, excessjitter will be created in the transmissions.

A higher rate EPON may be “asymmetric” when only the downlink rate isincreased, or “symmetric” when both the uplink and downlink rates areincreased. One way to implement a higher rate EPON is to use a 10 Gsolution, as expected in the next stage of EPON technology. Morepreferably, the present invention discloses an asymmetric solution thatuses a proprietary 2 G rate. This “2 G solution” can use some existing2.5 G SONET components, which translates to 2 G Ethernet after the 8b/10 B line code. Thus, in a particular case of asymmetric EPON,described in detail herein, the uplink rate is 1 G and the downlink rateis 1 G or 2 G.

With reference to a higher rate EPON of the present invention,exemplarily in FIG. 1, OLT 102 is operative to transmit at a doubledownlink rate (2 G ). While in every EPON there is only one logical OLT,the present invention discloses a logical 2 G OLT achieved through afunctional combination of two “normal” or legacy 1 G OLT “devices”.Hereinafter, a logical 2 G OLT of the present invention will be simplyreferred to as a “double rate” or 2 G OLT. In the higher rate EPON ofthe present invention, some of the ONUs (exemplarily 106 and 108) mayoperate at the normal 1 G rate (i.e. be “1 G devices”) while other ONUs(exemplarily 110) may operate at both normal and higher (2 G) rates, inwhich case they are called “2 G devices”. Each ONU has a correspondingreceiver operative to receive transmissions at the respective rate (notshown). A 1 G ONU includes a 1 G receiver and a 2 G ONU includes both a1 G receiver and a 2 G receiver, as it can work both at the basic rateand at the double rate. Similar to the 2 G functionality of a 2 G OLT,the 2 G functionality of a 2 G ONU may be achieved by using two 1 G ONUdevices.

It is to be understood that while the 2 G OLT and ONUs of the presentinvention are described as a functional combination of legacy 1 Gdevices, the various methods disclosed herein can be equally wellimplemented in dedicated 2 G devices. Such dedicated 2 G OLT and ONUspreferably include all the required functionalities to effect theimplementation of the methods.

In a preferred embodiment, the inventive operability of a higher rateEPON according to the present invention is facilitated by the subsystemsdescribed in FIGS. 2, 3 and 5 b.

FIG. 2 shows schematically a subsystem 200 of two 1 G OLT devicesfunctioning together to achieve a 2 G link. Exemplarily, subsystem 200may represent a logical 2 G OLT of the present invention. Subsystem 200comprises two 1 G OLT devices 202 and 204, a complex programmable logicdevice (CPLD) 206 and a transceiver 208, interconnected as shown. TheCPLD is operative to combine functionalities of two 1 G devices.Subsystem 200 supports a 2 G BW downlink transmission of a 2 G OLT usinglegacy (standard) OLT 1 G devices.

FIG. 3 shows schematically a subsystem 300 of two 1 G ONU devicesconnected together to achieve a 2 G link. Exemplarily, subsystem 300 mayrepresent a 2 G ONU of the present invention. Subsystem 300 comprisestwo 1 G ONUs 302 and 304, two CPLDs (CPLD A 306 and CPLD B 308), atransceiver 310, and a respective bidirectional PON interface 312, allinterconnected as shown. System 300 supports a 2 G BW downlinktransmission using legacy 1 G ONU devices. Each of subsystems 200 and300, separately or in combination, may be used to implement any of themethod embodiments of the present invention.

The standard 8 B/10 B line code is a very popular code well known in theart, originating from the fiber channel and adopted by IEEE for the802.3 standard. In a first embodiment of the method for bandwidthdoubling in an EPON according to the present invention (not shown in thefigures), the 8 B/10 B line code is used in the double-rate downlink ofan EPON network. In other words, a 2 G OLT of the present invention isconfigured to transmit at 2 G using the 8 B/10 B line code.

In a second embodiment of the method for bandwidth doubling in an EPONaccording to the present invention, the method uses interleaving of twodifferent 1 G links, combined to a single 2 G link, as shown in Table 1:

TABLE 1 M0 L0 M1 L1 M2 L2 M3 L3 M4 L4 M5 L5 M6 L6 M7 L7 A0 A1 A2 A3 A4A5 A6 A7

Table 1 shows the OLT-transmitted bit order. The top line is thetransmission bit order at the double rate. The bottom line is thetransmission bit order at the basic rate. Every byte includes 8 bits0-7. MO is the least significant bit (LSB) in byte M. L0 and A0 aresimilarly LSBs of two other respective bytes L and A. Byte A istransmitted at the basic rate, and bytes M and L are transmitted duringthe same time at the double rate. Exemplarily, the transmission isperformed using the two 1 G OLTs in system 200 in FIG. 2. CPLD 206 isresponsible for taking one bit from each 1 G OLT device, and placingthose one after the other on the speed-up link. M and L are alwaysspaced two bytes apart, as shown in Table 2, which shows an arbitrarysequence of a 4-byte transmission (P and T are different bytes):

TABLE 2 M P L T

The benefit of this embodiment of line code interleaving (i.e. theinterleaving of the two links) is that it prevents comma reception (seeIEEE802.3 clause 36) by a basic (1 G) rate receiver. The probability ofa false lock of the basic rate receiver with this scheme is similar towhite noise lock probability. The comma of the double rate is used alsofor a comma locking mechanism of the basic rate.

In yet another embodiment of the method for bandwidth doubling in anEPON according to the present invention, shown in FIG. 4, theinter-packet gap (IPG) length in the double-rate transmission isdoubled, to guarantee comma synchronization at the basic rate.

FIG. 4 shows an embodiment of packets in a transmission spaced by adouble IPG length in order to allow simultaneous comma synchronizationat two rates. The figure shows the transmission of two packets 400 and402. An IPG 404 is doubled over a standard (normal) IPG 406, therebyachieving simultaneous comma synchronization between the 1 G and 2 Gdownlinks.

In yet another embodiment of the method for bandwidth doubling in anEPON according to the present invention, support for a mixed network inwhich 1 G and 2 G devices operate simultaneously is provided by definingwindows of transmission. Each window's transmission occurs at a definedrate. To support this, the downlink transmission needs to includereordering, i.e. grouping of downlink packets of different rates tominimize the number of transitions between the two different rates. Inreordering, the packet transmission order is not necessarily the packetarrival order to the OLT. Reordering also requires consideration of therate of the destination device (ONU).

FIG. 5 a shows schematically a reordering flow chart, and FIG. 5 b showschematically a subsystem implementing the reordering. The flow chart isdivided into two parts: an ingress process (steps 500-504), in which apacket enters a queue, and an egress process (steps 550-558), in whichthe packet leaves the queue. The ingress process in performed in aningress process module 580, storage for packets to be transmitted at thebasic rate is performed in a basic rate packet storage module 582,storage for packets to be transmitted at the fast rate is performed in afast rate storage module 584, and egress selection is performed in anegress selection module 586. Modules 580-586 could exemplarily beimplemented in a CPLD or in a dedicated 2 G OLT. Exemplarily, modules580-586 may be implemented in OLT 102 in FIG. 1.

In the ingress process, a comparison step 500 checks if the incomingpacket should be transmitted at the normal or at the double rate. If atthe normal rate, the packet is placed in a normal rate queue in step502. If at the double rate, the packet is placed in a double rate queuein step 504.

The egress process is responsible for selecting a group of queues fortransmission. A check in step 550 establishes if a packet is pending ateither queue. If no packet was pending previously and the transmissionline is idle, a newly pending packet is transmitted immediately in thedownstream direction in step 552. Otherwise, the history of previouspacket transmission plays a role in the selection of the nexttransmission. A check in step 554 establishes if a previouslytransmitted packet was transmitted at a normal or at a double rate. Ifat a normal rate, weights (α_(normal), α_(double)) for selecting thequeue groups are assigned in step 556. Typically,α_(normal)>>α_(double). Otherwise, if the previously transmitted packetwas transmitted at the double rate, the weights (β_(normal), β_(double))for selecting the queue groups are assigned in step 558. Typically,β_(normal)>>β_(double).

The weights are used for the selection process as a mean to prefer onequeue over the other. They represent an abstract mechanism for creatinga preference. The selection occurs at the OLT. With this scheme, thebasic rate receiver is not required to remain locked on the fast(double) rate clock. The beginning of the basic rate transmission willinclude a long IPG assisting locking of the basic rate receiver,described exemplarily in FIG. 4. When a packet from a queue is chosenfor transmission, if the basic rate transceiver cannot lock on thedouble rate, then each fast rate transition will cause loss ofsynchronization. Therefore, some overhead is required for regainingsynchronization before a transmission at the basic rate.

In yet another embodiment of the method for bandwidth doubling in anEPON according to the present invention, the entire 8 B/10 B code isremoved from just the downlink transmission. In this case, the effectivebandwidth is increased by 25% because of the removal of the line codeoverhead. In case of the double rate transmission, the rate willincrease from 2 G to 2.5 G.

In yet another embodiment of the method for bandwidth doubling in anEPON according to the present invention, the method usessymbol-by-symbol multiplexing (SSM), in which in every time unit asymbol from a first channel is transmitted followed by a symbol from asecond channel. The receiving side performs de-multiplexing. Eachreceiver receives the information simultaneously and instantaneouslywith the other receiver. Consequently, the utilization of existingchannel transmitters and receivers is straightforward. The SSM enablesreconstruction of the original packet transmission time and use ofexisting devices for interleaving two channels. The dual-rate operationsupports a solution based on concatenation of 1 G devices e.g. as shownin FIGS. 2 and 3. Table 3 shows an exemplary transmitted bit order usingSSM. The same bit order as in Table 1 is used. M is the currentlytransmitted byte of channel A, and L is the currently transmitted byteof channel B.

TABLE 3 M0 M1 M2 M3 M4 M5 M6 M7 L0 L1 L2 L3 L4 L5 L6 L7

In yet another embodiment of the method for bandwidth doubling in anEPON according to the present invention, the rate is doubled throughpacket-by-packet multiplexing (PPM). PPM is based on taking packets fromeach channel on a round-robin basis. However, packets may have differentlengths, and packet arrival timing needs to be maintained.

FIG. 6 shows a basic PPM system 600 comprising two channels, 1 and 2,operative to send information packets from two 1 G OLT devices 604 and606 (exemplarily similar to OLT devices 202 and 204 in FIG. 2) throughtwo first-in first-out (FIFO) buffers FIFO 1 and FIFO 2 into amultiplexer (MUX) 602. The multiplexer selects the FIFO for transmissionbased on the fullest FIFO (a FIFO check is done by the MUX), and outputsa 2 G downstream traffic. The two FIFOs and the MUX may be exemplarilyimplemented in CPLD 206 in FIG. 2. An illustration of PPM is shown inFIG. 7.

In FIG. 7 a, a packet originated from a first 1 G link 702 is marked asPA and a packet originated from a second 1 G link 704 is marked as PB.The numbers attached to each PA or PB packet indicate the packetsequential number, i.e. its timing. A first arrow 706 converts the twostreams of packets into a combined 2 G stream 708. This may be doneexemplarily by MUX 602 in FIG. 6. As seen, the original timing of apacket (indicated by the relative timing between packets) is modified,and consequently the channel timing property is ruined. A second arrow710 reconstructs the 2 G stream into two 1 G streams 712 and 714. Thereconstruction may be done exemplarily in CPLD 306 of FIG. 3. Note thatwhile it is desired that the original packet order be preserved, in thisexample the reconstructed packet error is different than the originalone. To solve this problem, i.e. in order to make the PPM preserve theoriginal (1 G ) packet order, time information is added to each packet.The time information contains a “packet delay” between a first bytearrival from each original (1 G) channel link (OLT devices 604 and 606in FIG. 6) and the time the first byte leaves the speed-up (2 G) channelat the MUX 602 output, in FIG. 6. Using this information, a receivingdemultiplexer in CPLD 306 in FIG. 3 delays the transmission of eachpacket by the amount of (MAX delay-packet delay) from the time thepacket was received in the original 2 G channel (input to transceiver310 in FIG. 3) to the time the packet leaves toward a specific channel(to ONUs 302 and 304 in FIG. 3). “MAX delay” is a constant, preferablyset to be greater than the maximal packet length (MTU), i.e. MAXdelay>packet MTU. The result, shown in FIG. 7 b in reconstructed links762 and 764, is to restore the original packet order. The maximal delay,i.e. the overall delay between packet arrival to the OLT until packetreception by an ONU is equal to the length of the two longest packets inthe 1 G channel, or of the four longest packets in the 2 G channel.Exemplarily in FIGS. 7 a,b, this refers to packets transmitted inchannel 752 and packets reconstructed in channel 762.

The time information is added as a side-band (side information) to eachtransmitted packet. For example, in Ethernet, 2 bytes in the 8 bytepreamble of each packet can be used for this purpose. These values areignored in a standard Ethernet receiver, and are overridden herein for anew purpose. These bytes will be returned to the original value (0×55)after the DEMUX will use the inserted time information

An error correction function can optionally be placed in the preamble toassist the operation. Preferably, a single bit is corrected. Anexemplary summary of 2 bytes placed in the preamble is shown in FIG. 8.

The broadcast behavior of a mixed 2 G EPON is described next.Broadcast/multicast is easily supported in a unified network, in whichall devices are configured to work at a single rate. In a unifiednetwork, a packet is transmitted only once, in contrast with a mixed(dual-rate) network, where the packet must be transmitted twice, once ateach rate. A discovery protocol can be configured to be performed onlyat the normal rate.

In such a unified network, the rate of downlink transmission from an OLTto each ONU can be increased or decreased by negotiation. Both the ONUand OLT must agree on the used rate. Negotiation can begin by either theONU or the OLT as an initiator. Upon acknowledgement from the ONU(regardless of the initiator), the OLT starts to use the new downlinkrate. Upon acknowledgement from the OLT (regardless of the initiator),the ONU starts to use the new uplink rate.

With respect to the handling of the clock rate in each of theembodiments of the method for bandwidth doubling in an EPON according tothe present invention, the clock rate can be detected by attempting tolock the clock and data recovery (CDR) on the highest rate clock. If thecomma is not locked, the CDR is shifted to a lower rate. In other words,this is a method with two phases. The first phase is initialization. Thereceiver tries to lock on one frequency. If it fails it, moves to thesecond phase, where the receiver attempts to lock on the otherfrequency. Following failure, this process repeats itself by returningto the first phase until lock is achieved. To maximize resemblance tobasic 1 G EPON and since uplink transmission is not modified, the basictime unit of MPCP, called Time Quanta, or TQ, is also not modified.

All publications, patents and patent applications mentioned in thisspecification are herein incorporated in their entirety by referenceinto the specification, to the same extent as if each individualpublication, patent or patent application was specifically andindividually indicated to be incorporated herein by reference. Inaddition, citation or identification of any reference in thisapplication shall not be construed as an admission that such referenceis available as prior art to the present invention.

While the invention has been described with respect to a limited numberof embodiments, it will be appreciated that many variations,modifications and other applications of the invention may be made.

1. A method for bandwidth doubling in an Ethernet passive opticalnetwork (EPON) comprising the steps of: a. providing a mixed system inwhich a logical optical line terminal (OLT) is optically coupled to atleast one single rate optical network unit (ONU) and to at least onedouble rate ONU, wherein the OLT is operative to transmit downstreampackets through a downlink to each ONU, and wherein each ONU isoperative to transmit upstream packets through an uplink to the OLT; andb. enabling the OLT to transmit to each ONU at a downstream rate thatmatches the respective ONU rate.
 2. The method of claim 1, wherein thestep of providing a mixed system in which a logical OLT is opticallycoupled to at least one single rate ONU and to at least one double rateONU includes providing a mixed system in which each single rate ONU is a1 Gigabit per second (1 G) ONU and each double rate ONU is a 2 G ONUoperative to perform combined 1 G and 2 G operations.
 3. The method ofclaim 2, wherein the step of providing a mixed system in which a logicalOLT is optically coupled to at least one single rate ONU and to at leastone double rate ONU includes providing a mixed system in which thelogical OLT is a 2 G OLT that comprises a functional combination of two1 G OLT devices and is operative to transmit to each 2 G ONU at a 2 Grate.
 4. The method of claim 3, wherein the step of enabling the 2 G OLTto transmit to each 2 G ONU at a 2 G rate includes configuring the OLTto transmit to each 2 G ONU at a 2 G rate using a 8 B/10 B line code. 5.A system for effecting bandwidth doubling in an Ethernet passive opticalnetwork (EPON) that comprises a logical optical line terminal (OLT)optically coupled to a plurality of optical network units (ONUs), thesystem comprising a mechanism, for doubling the downlink transmissionrate from the OLT to at least one ONU configured to receive double ratetransmissions.
 6. The system of claim 5, wherein the mechanism fordoubling the downlink transmission rate from the OLT to at least one ONUcomprises a subsystem comprising two single rate OLT devicesfunctionally connected through a programmable logic device to provide adouble rate transmission functionality to the logical OLT.
 7. The systemof claim 6, wherein the single rate is a 1 Gigabit per second (1 G) rateand wherein the double rate is a 2 G rate.
 8. The system of claim 7,wherein the at least one ONU configured to receive double ratetransmissions is a 2 G ONU that comprises two 1 G ONU devicesfunctionally connected to receive 2 G downlink transmissions.
 9. Thesystem of claim 6, wherein said double rate transmission functionalityincludes symbol-by-symbol multiplexing.
 10. The system of claim 5,wherein said programmable logic device is a complex programmable logicdevice.
 11. A method for bandwidth doubling in an Ethernet passiveoptical network (EPON) comprising the steps of: a. providing a mixedsystem in which a logical optical line terminal (OLT) is opticallycoupled to at least one single rate optical network unit (ONU) and to atleast one double rate ONU that is operative to perform combined singlerate and double rate operations, wherein the OLT is operative totransmit downstream packets through a downlink to each ONU, wherein eachONU is operative to transmit upstream packets through an uplink to theOLT, and wherein the logical OLT is a double rate OLT that comprises afunctional combination of two single rate OLT devices and is operativeto transmit to each double rate ONU at a double rate; and b. enablingthe OLT to transmit to each ONU at a downstream rate that matches therespective ONU rate; wherein the step of enabling the double rate OLT totransmit to each double rate ONU at a 2 G rate includes symbol-by-symbolmultiplexing.
 12. The method of claim 1, wherein the EPON is a unifiedEPON.
 13. The system of claim 5, wherein the EPON is a unified EPON. 14.The method of claim 11, wherein the EPON is a unified EPON.
 15. Themethod of claim 9, wherein said symbols are different from packets thatare transmitted by the OLT to the at least one ONU.
 16. The method ofclaim 11, wherein said symbols are different from said downstreampackets.